1. Field of the Invention
This invention relates generally to an apparatus and method for underground pipe installation and, more particularly to an apparatus and method for trenchless installation of product pipe through pilot-tube guided, auger boring.
2. Discussion of Related Art
Various applications require the installation of underground pipe between two stations without creating a trench therebetween, such as between two manholes. Techniques have been developed to accomplish such trenchless installation of pipe. Most of these methods are based on pipe-jacking principles, that is, protective pipes or product pipes are jacked through the ground by means of a hydraulically operated jacking unit from a previously prepared starting or jacking shaft to a target shaft. For example, one method for installing product pipes of non-man-sized nominal diameters in such an environment is shown and described in U.S. Pat. No. 4,630,967 and DE-GmS No. 82-05-543, the teachings of which are incorporated herein by reference.
Microtunneling involves horizontal jacking systems directly installing product pipe by jacking the pipe to a predetermined line and level. This technique for the trenchless construction of service tunnels is accurate for non-man size nominal inside diameter pipe and is accurate for drive lengths of about 1000 linear feet. Tunnel boring techniques install product pipe by jacking pipe into a bore created by a cutting head. This method is used for pipes with man-size nominal inside diameter (e.g., 36 in. and larger) and is accurate for drive lengths of approximately 2000 linear feet. Both microtunneling and tunnel boring techniques typically employ laser steering. For large diameter pipes (up to 72 in. internal diameter), auger boring techniques using a horizontal earth boring machine can be used for short drives up to approximately 300 linear feet. The line and grade control systems for auger boring, however, is not very precise.
To reduce the load encountered by the product pipe in these techniques and to thereby drive greater lengths, a two-pass system may be employed. In this system, a temporary steel liner is installed initially into the bore to the predetermined line and level. After the bore is completed, product pipe is jacked into the bore replacing the temporary pipe. Two-pass systems allow greater load, reduce damage to the permanent pipe and achieve longer drives. But, these advantages may be offset due the additional time required for operation.
The pilot-bore method of microtunneling overcomes this concern by employing a three-pass system. In the first stage shown in FIG. 1, precision on-target jacking of pilot pipes displaces the soil by consolidation to create a bore. This method is particularly designed for softer soils where consolidation is possible without creating problems associated with soil movement. Suitable soils are cohesive soils, such as clays and silts with N values up to 60, as well as sands.
The pilot pipes are kept on-line through a steering mechanism including a theodolite fitted with a camera and an illuminated target displayed on a monitor, which is viewed from the jacking shaft. Any deviation from the reference axis is correctable through counter steering of the pilot-pipe assembly.
After the pilot pipe has been installed in the length required and has entered the target shaft, a second stage of the pilot-bore method widens the bore to a specified diameter by means of an expanding head, as shown in FIG. 2. The expanding head has a diameter greater than the pilot pipes' diameter and is fitted on both a first segment of a steel sleeve assembly and attached to the last section of pilot pipe in the jacking shaft. A driving unit advances the first steel sleeve segment and expanding head through the pilot-pipe created bore, with the pilot-pipe assembly serving as guidance to maintain the required line and level. Additional steel sleeve segments are advanced one behind the other into the drive unit to urge the pilot tube from the newly created bore segment by segment. The expanding head is typically an excavating type whereby spoil may be removed internally of the sleeves As the expanding head is advanced and cuts the soil to the required diameter, an auger chain positioned within the steel sleeves removes the displaced soil to a discharge positioned in the jacking station.
As the expanding head enters the target shaft, the third stage of the pilot-bore method shown in FIG. 3 involves placing product pipe into the drive unit and jacking it into the bore one segment behind the other to advance the steel sleeves of the second stage through to the target shaft. As such, product pipe is placed in the desired position without disturbing the surface of the earth between the jacking station and target station. The pilot pipe and steel sleeves are recovered segment by segment and ready to use for the next installation.
Pilot-bore microtunnelling methods have been used to install product pipes of various kinds with internal diameters in the range from 150 mm to 450 mm.
A problem with the pilot-bore microtunnelling method is that it is limited in the maximum diameter of pipe installable (to approximately 575 mm.). For most soil types and conditions, the maximum outer diameter of the expanding head, which determines the maximum diameter of the permanent piping that follows the expanding assembly is about 221/2 in. As the diameter of the expanding head begins to exceed this diameter, the pilot tube, which advances through the soil by consolidation to create a bore having walls formed of compacted soil, looses its ability to steer the trailing expanding head along a desired line and grade to the intended target. When the expanding head diameter is about 221/2 in., the compacted soil walls of the bore hole possess sufficient strength to resist a deflection of the pilot tube passing through the bore hole, which deflection would otherwise be caused by uneven forces imposed on the leading edge of the expanding head as it encounters and advances through varying soil types and conditions. If the pilot tube deflects from the desired line and grade as it is pushed through the bore hole by the expanding head, then the expanding head, and more importantly, the permanent piping following behind the expanding head, will be steered off course.
The maximum diameter of the expanding head for successful use in trenchless tunneling is, therefore, a function of strength of the pilot bore hole walls, which in turn is effected by the degree of compaction caused by the pilot tube as it is initially advanced through the soil. That is, pilot tubes having larger diameters will create a higher degree of soil compaction, and thus greater wall strength, as the pilot tube advances through the soil. Soil type and soil condition will also effect the degree of soil compaction.
The resistance to deflection is further dependent on the circumferential contact area of the pilot tube. Thus, pilot tubes having a diameter greater than 41/2 in. would permit the expanding head to have a diameter larger than about 221/2 inches. Pilot tubes larger than 41/2 in. are typically not installed by consolidation because of potential damage to surrounding structures and utilities due to soil movement during the installation process. Experience has shown that pilot tube assemblies with pilot tubes having about 41/2 in. diameters provide optimal results for typical soil and work conditions.
In some applications, capacity requirements between two manholes may require the use of pipe having a much larger diameter, such as 40 inches or larger. And, these applications may require that the job be completed quickly and efficiently. A need therefore exists for a method and apparatus for trenchless installation of product pipe, which is not labor intensive and allows for man-sized nominal diameter pipes to be jacked to a predetermined line and level. The present invention addresses these problems by employing a pilot-tube guided, auger boring technique that greatly reduces the torque imparted to large diameter pipes and further permits product pipe to be installed concurrently with the expanding member of the pilot-bore methods. As such, the method of the present invention is not limited to the approximately 575 mm. diameter pipe but, instead can be used to install 1000 mm. (40-in.) or larger diameter pipe.